1. Field of the Invention
The present invention relates to a time base corrector to be used for correcting time base variations included in reproduced signals of video tape recorders (VTR's) and laser disk players.
2. Description of the Related Art
There have been various methods which are used for a time base corrector. For example, in the U.S. Pat. No. 3,931,638, time base instability of signals is corrected by use of paired analog shift registers. While an input signal is being written in one of the shift registers by using a clock at a rate corresponding to its time base instability, the other shift register is read out at a uniform rate.
A time base corrector (hereinafter referred to as a TBC) using a variable delay line as other type of TBC is described in the "Overview of Time-Base Correction Techniques and Their Applications" by K. Sadashige, SMPTE Journal Vol. 85 Oct. 1976, pp. 781-791.
A digital TBC using a digital memory is also available as one of the main modern TBC's. An example of this type of TBC is found as disclosed in the U.S. Pat. No. 4,120,000. According to the TBC of this type, a video signal from a VTR is A/D converted by using a clock rate which follows a detected time base error and the A/D converted signal is written into the memory by using the same clock rate so that the time base error is corrected. The signal is read from the memory by using a clock which is obtained by phase modulating a reference clock in accordance with the quantity of a velocity error. The velocity error is corrected to obtain an output signal free from time base variations by D/A converting the signal which has been read from the memory by using the same clock.
According to the above method, however, the digital data before the D/A converter includes a velocity error. Therefore, there arises such an inconvenience that an attempt to carry out other processing in the state of digital data before the D/a converter will not be successful.
In order to solve the above problem, there is a TBC which uses an adaptive interpolation filter, as disclosed in the U.S. Pat. No. 4,905,101. This TBC corrects time base errors in a digital signal with an interpolation filter after an input signal has been converted into a digital signal in a predetermined sampling period.
However, the structure of absorbing such a large time base error as a skew in this method can not be said to be an idle one from the viewpoint of its circuit scale.